This invention relates generally to magnetic resonance imaging (MRI) utilizing nuclear magnetic resonance (NMR) phenomena. It more particularly relates to control of the driver circuits for gradient coils and/or other electromagnet coils utilized to supplement the nominally static main magnetic field B.sub.o of a main magnet structure.
This invention may be considered as generally related to at least the following prior, commonly assigned, U.S. patents and/or patent applications:
1. U.S. Pat. No. 4,755,755 issued Jul. 5, 1988 to Carlson entitled "Compact Transverse Magnetic Gradient Coils and Dimensioning Method Therefor";
2. U.S. Pat. No. 4,829,252--Kaufman issued May 9, 1989 entitled "MRI System With Open Access to Patient Image Volume";
3. U.S. Pat. No. 4,885,542--Yao et al, issued Dec. 5, 1989 entitled "MRI Compensated for Spurious NMR Frequency/Phase Shifts Caused by Spurious Changes in Magnetic Fields During NMR Data Measurement Processes";
4. U.S. Pat. No. 4,970,457--Kaufman et al issued Nov. 13, 1990 entitled "MRI Compensated for Spurious Rapid Variations and Static Magnetic Field During a Single MRI Sequence";
5. U.S. patent application Ser. No. 07/659,181 filed Feb. 22, 1991 now U.S. Pat. No. 5,157,330 by Kaufman et al entitled "Method and Apparatus For Compensating Magnetic Field Inhomogeneity Artifact in MRI";
6. U.S. patent application Ser. No. 07/702,428 now pending by Carlson et al filed May 20, 1991 (as a CIP of Ser. No., 07/688,131 filed Apr. 19, 1991 now pending which is, in turn a CIP of Ser. No. 07/686,622 filed Apr. 18, 1991 now pending) entitled "Apparatus and Method for Stabilizing the Background Magnetic Field During MRI"; and
7. U.S. patent application No. 07/705,964 now pending by Kaufman et al filed May 28, 1991 entitled "NMR Relaxometry Using Fixed RF Frequency Band."
The entire content of each of the above referenced related U.S. patents and patent applications is hereby incorporated by reference.
Magnetic resonance imaging systems are now commercially available from a number of sources. There are, in general, several techniques known to those in the art. Some exemplary MRI techniques are set forth, for example, in commonly assigned issued U.S. Pat. Nos. 4,297,637; 4,318,043; 4,471,305 and 4,599,565. The content of these issued patents is hereby incorporated by reference.
In all MRI systems now commonly used, a main magnet structure is used to provide a substantially uniform homogeneous magnetic field within a patient image volume along a predetermined axis (e.g., the z axis of the usual x,y,z orthogonal coordinate system). When precisely controlled gradient magnetic fields are superimposed within the image volume with respect to various different axes, the detectable NMR characteristics of NMR nuclei can be spatially encoded (in conjunction with suitable RF nutation pulses) so as to produce RF responses that can be detected and processed to provide two dimensional arrays of display pixel values (representing two and/or three dimensional depictions of NMR nuclei within the patient image volume). However, the accuracy of the MRI process is directly related to the degree of homogeneity in the static field and the degree of linearity in imposed gradient fields along the selected axes (the gradient field ideally being a linear function of position with respect to one axis and a constant as a function of position with respect to other orthogonal axes within the patient volume). To the degree that these desired goals of static field homogeneity and of gradient field linearity along precisely selected axes fail to be attained, then inaccuracies and/or artifacts in the resulting displayed image can be expected. Accordingly, considerable effort has been previously expended toward the ends of either attaining these goals or in correcting or compensating for deviations from such goals.
Some MRI systems have main magnet structures that include permanent magnets, iron and/or other ferromagnetic elements in the relevant main magnet magnetic circuit. For example, a relatively low field open architecture main magnet is employed in the MRI system design described by related U.S. Pat. No. 4,829,252 referenced above. In such structures, the ferromagnetic components exhibit remanent magnetization and hysteresis effects as a result of past magnetization history. This can cause a number of undesirable effects such as image distortion, intensity variations, artifacts of various sorts, etc. Superconducting magnets may also include iron flux return elements exhibiting hysteresis.
Remanent magnetization and hysteresis effects are especially troublesome with with respect to the rapid sequentially changing energization of gradient electromagnet coils. For example, the usual phase encoding gradient coils (e.g., oriented to produce a gradient in the y axis dimension) typically are sequenced in 256 steps from a drive pulse of maximum positive value to a maximum negative value (e.g., over 256 successive MRI data gathering cycles). The first large pulse will leave a relatively large remanent magnetization throughout the next 127 successively smaller magnitude positive drive pulses. However, as the drive pulse polarity reverses, the remanent gradient will also be caused to be reversed and will then stepwise increase in magnitude throughout the remainder of the complete imaging sequence to a maximum negative value. Furthermore, even for gradient axes which do not change in magnitude or polarity during the entire imaging sequence (e.g., as along the x axis which is typically energized during NMR RF signal readout), a remanent gradient of some sort will remain to possibly distort a subsequent imaging sequence.
Ambient or other environmental changes can also cause undesirable changes in the magnetic field of an MRI system. For example: local changes in the earth's magnetic field or local magnetic field changes induced by local movements of large magnetically permeable masses (e.g., elevators, locomotives, etc) by the passage of large local electrical currents and their associated magnetic fields (e.g., as in the drive circuits of elevators, trolley cars, trains, subways, etc.), by ambient temperature changes and related changes in magnetic circuit properties, hysteresis effects in magnetically coupled bodies. These are all potential sources of undesired deviations from the optimum spatial distribution of magnetic field orientation and strength within the patient image volume of an MRI system throughout any given imaging procedure--or over the historical period of system installation at a given site or ambient conditions as compared between different sites. The latter may impair the ability to accurately compare images taken at the same site at widely separated intervals of time.
Currently substantial efforts are required during installation of an MRI system at a particular site in an attempt to minimize such difficulties. Extra care must currently also be taken to assure quality and repeatability in magnet production for MRI systems. Many special processing steps or other precautions are often also required to provide reliability and image quality with sufficiently high standards in view of these ongoing problems. Accordingly, there is considerable need for a more comprehensive and efficient technique to further minimize the possible adverse effects of such potential problems.
Other potential sources of similar problems are eddy currents generated by rapidly changing magnetic gradient fields in surrounding electrically conductive materials. Associated with each attempted change in the magnetic gradient flux will be the generation of eddy currents in any nearby unshielded conductors which, in conformance with Lenz's law, will produce magnetic fields which oppose the attempted change in the gradient field. Accordingly, it has long been known that some kind of eddy current compensation must be included in the drive current supplied to a gradient coil.
In other words, consistent, reliable operation of a magnetic resonance imaging system relies strongly on the creation of nearly ideal gradient flux pulses inside the volume to be imaged. Nearby electrically conductive structures inherently support eddy current loops when exposed to the rapidly switched gradient fields and these result in various distortions to the desired spatial distribution of magnetic flux. Such eddy currents, located in various nearby metallic structures, decay in a manner that is characteristic of a collection of somewhat different exponential time constants. If not compensated, the related time variation produced in the net magnetic flux actually present within the patient image volume would be sufficiently severe to distort section profile and end-plane resolution of the imaging system.
As a consequence, magnetic resonance images have long used some kind of compensation to reduce the effect of such secondary "eddy" currents. The most common prior technique is an open loop feedback system whereby the gradient flux demand pulse is purposely initially overdriven (e.g., "pre-emphasized" in a predetermined and pre-calibrated wave form). Determining the exact characteristics of such overdriving for a particular installation site presently requires a considerable and lengthy effort. One prior approach towards automation of this process is set forth in U.S. Pat. No. 4,928,063, dated May 22, 1990, entitled "Automatic Eddy Current Correction" naming David L. Lampman et al as inventors. Hopefully, once this laborious process has been completed, the open loop control system will overdrive the gradient coil in just the right manner to thereafter anticipate all induced eddy currents and to result in a net actual flux field that approximates the ideal.
However, not only does this kind of conventional system setup consume considerable time initially (and thereafter in a maintenance mode), it is virtually impossible to find one predetermined overdrive specification that will properly compensate for eddy currents under all subsequent changing operational conditions. For example, if the magnet structure is a cryogenic superconducting magnet then, as the cryogen boils off, the temperature of various metallic conductor elements varies which, in turn, causes a significant change in resistivity and a noticeable change in the time response of eddy current subsystems. Future magnet designs may eliminate the baths of cryogens and rely on continuous cooling by external refrigerators. Such designs may have intrinsically larger variations in eddy current behavior with the cycling of the refrigerator.
Furthermore, spatial variations in eddy current fields often do not exactly track the gradient coil flux field. The additional current needed to compensate for eddy currents is a priori dependent on spatial position. Therefore, one cannot successfully completely eliminate eddy current effects in an entire region of space by the open loop compensation of a single coil.
In short, it is virtually impossible for a simple open loop compensation system to exactly correct for eddy current effects. A typical overdrive compensation involves a current overshoot of approximately 20% with a decay to an asymptotic "steady state" value involving two or three time constants--plus a similar undershoot when the drive pulse turns "off," and with a similar multi-time constant decay to the asymptotic current state.
A less common but somewhat better technique for reducing adverse eddy current effects is to wind a shield coil around the gradient coils. Although this may substantially eliminate the effect of some eddy currents (e.g., those induced in the aluminum cryogenic container), it occupies a considerable additional portion of the available magnet bore space thus substantially decreasing access to the image volume while adding substantial cost, weight, etc to the overall MRI system.
As a part of the lengthy setup procedure now required for installation of an MRI system at a particular site, considerable effort is often given to centering the gradient coils in an attempt to avoid asymmetric eddy current effects. If the eddy currents are substantially asymmetric, then there may be no technique known in the prior art for adequately compensating them.